Natural gas is often discovered and produced in locations that are remote from where the gas can be marketed and distributed to end users. When suitable pipelines are available, the natural gas can be transported to market in either a gaseous or liquid form, however, there are many instances in which such pipelines are not available or practical for connecting a particular natural gas supply with consumers. When natural gas supplies are located overseas or a substantial distance from a suitable distribution system, it may be necessary to transport the gas by vessel. Such vessels typically include specially designed carriers that transport natural gas as a liquid housed in large insulated containers or tanks.
When transported at or near atmospheric pressure liquefied natural gas (LNG) is held at temperatures slightly below about −160° C. This temperature represents the boiling-point temperature for methane at atmospheric pressure. However, since the composition of natural gas will typically contain variable amounts of heavier and higher boiling hydrocarbons such as ethane, propane, butane and the like, the liquefied gas will be characterized by a somewhat higher boiling temperature, usually ranging from about −151° C. to about −164° C. depending upon composition. At or near a destination, the LNG must be regasified and warmed before it can be introduced into a distribution pipeline. In addition, depending on the requirements of the pipeline and local natural gas specifications, the LNG may be pressurized, depressurized, blended, odorized or subjected to other processing before it can be introduced into a pipeline or similar distribution system.
In both the loading and off-loading of LNG or other cryogenic fluids from a vessel, loading arm(s) and flow line(s) are used to transfer the cryogenic fluid. Due to the relatively low temperature of these fluids, the loading arms and flow lines must be pre-cooled or chilled to cryogenic temperatures before transfer operations can begin. Conventional cool-down procedures can require two to five hours depending on the materials and features of the arm and flow lines, the port requirements, and the recommendations of the loading arm/flow line manufacturer. Modifications that would enable such cool-down procedures to be completed more quickly while complying with port requirements and manufacturer recommendations would be advantageous and would enable additional vessels to be loaded and unloaded at a given terminal each year.